Various transmissions are known in the art and are commonly used to transfer motive power received at the input and transferred to the output of the transmission. In general, the power is transmitted from an engine coupled to the input to a live axle coupled to the output. The transmission often converts speed and torque, and may perform an up-conversion or a down-conversion, wherein the output is of higher or lower speed than the input, while the torque is lower or higher than the input, respectively.
The terms “gears”, “sprockets” and “transmissions” are sometimes confused in informal use. In this specification, they will be used in the following way:
A gear is a toothed wheel or shaft designed to mesh with other gears for the purpose of transferring rotary motion or power. A sprocket is a toothed wheel designed to mesh with a flexible chain or belt for the purpose of transferring rotary motion of power. A belt is an elongated mechanical elastic element designed to carry force along its length and able to deform due to inherent flexibility of its material. A timing belt is a special elastic belt with a strong, high tensile fiber core band(s) and elastic teeth that can mesh into a sprocket, often used for precision motion and/or synchronization transfer such as in the valve timing mechanism of a car. A chain is an elongated mechanical piece-wise rigid element designed to carry force along its length and able to deform due to hinged connections between its pieces, called links. A transmission is a mechanical assembly comprised of any of gears, sprockets, belts, chains and other elements, whose primary purpose is transferring power between two mechanical interfaces such shafts. A drive is a combination of motor/engine and transmission.
There are several metrics for the evaluation of a transmission:
Efficiency is the measure of the power extracted from the driven (output) interface divided by the power delivered into the input interface. The torque limit is the maximum torque (at a specific interface) the transmission can handle before failure or accelerated wear. Backlash refers to the clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. It is generally characterized by the amount of free play possible in one interface while the other interface or interfaces are held immobile. Rigidity refers to the ability of the transmission to transfer force without incurring elastic deformation.
Harmonic transmissions use a combination of an externally-toothed inner gear [10] and a slightly larger internally-toothed outer gear [11]. The teeth of the inner gear [10] and outer gear [11] are of similar size. Harmonic transmissions operate by progressively engage successive teeth of the gears to achieve relative motion between them, and are able to withstand particularly high torques with low backlash. Harmonic transmissions often use elasticity to achieve the selective engagement of teeth. In such transmissions, the inner gear [10] is flexible and is deformed by an elliptical wave generator [12] through a flexible ball bearing [13] to progressively engage and disengage successive regions of itself from the rigid outer gear [11]. The design of a long-lived flexible gear is one of the more difficult aspects of such a transmissions. Harmonic transmissions provide large reduction ratios and can transfer large torques.
Planetary transmissions (FIG. 2) use a combination of an externally-toothed inner gear [20], an internally toothed outer gear [21], and a set of small mediating externally-toothed planetary gears [22] which are mounted and can free-spin on a common rotating carrier cage [23]. The cage and inner and outer gears are each connected to a shaft, resulting in a differential transmission. When both the outer and inner gears are of a similar diameter and the planet gears are small in comparison to them, the reduction ratio of a planetary transmission is very limited.
For further related information, the reader should also refer to U.S. Pat. No. 7,086,309, which describes a planetary harmonic motor.
Generally, the requirements of transmission performance is to withstand high moment (torque) in an axial direction and in a direction orthogonal to the axis of rotation, and have high rigidity and no backlash. For example, in a pair of gears, backlash is the amount of clearance between mated gear teeth. The backlash requirements may depends on allowance for lubrication, manufacturing errors, deflection under load, thermal expansion, and costs. When the specification of the transmission require high torque, high rigidity and no or minimal backlash, it leads to tight tolerances on the fabrication of the toothed wheels and the bearing supporting the rotating parts of the transmission. This, of course, increases the cost of the transmission. Therefore, what is needed is a transmission that provides high performance in terms of torque, rigidity and backlash, but that can be produced with lower tolerances and thus lower cost.